The assignee of this invention pioneered the development and application of methods and apparatus for foaming hot melt thermoplastic adhesives or so-called "hot melts" widely used throughout the industry for adhering many diverse products, as well as polymeric coatings and paints.
With respect to hot melt adhesives, for example, the assignee of this invention discovered that the adhesive strength of a bond achieved with a given volume of a selected hot melt adhesive could be appreciably improved and in most instances at least doubled if the adhesive were applied as a foam rather than as a conventional non-foamed adhesive. A hot melt thermoplastic adhesive foam system is disclosed in U.S. Pat. No. 4,059,466 of Scholl et al wherein a solid mixture of hot melt thermoplastic adhesive and blowing agent is heated and melted in a heated reservoir at a temperature above the melting temperature of the adhesive but below the decomposition temperature of the blowing agent. The molten adhesive and solid blowing agent mixture is then pressurized by a gear pump and supplied under pressure as, for example, 300 pounds per square inch, to a hot melt dispenser. Between the pump and the outlet of the hot melt dispenser, the molten adhesive and solid blowing agent mixture is further heated to a higher temperature at which the blowing agent decomposes and evolves a gas as, for example, nitrogen, which, at that pressure goes into solution with the liquid adhesive. The pressurized liquid/gas adhesive solution is then supplied to a valved type outlet at the adhesive dispenser from which the adhesive is dispensed at atmospheric pressure. Upon emerging from the outlet nozzle of the dispenser, the gas evolves from the solution in the form of small bubbles causing the adhesive to expand volumetrically. The resultant adhesive in an uncompressed state sets up as a homogeneous solid foam having gas cells substantially evenly distributed throughout the adhesive.
In U.S. Pat. No. 4,059,714 of Scholl et al, there is disclosed another hot melt thermoplastic adhesive foam system wherein the molten adhesive is mixed with a gas and pressurized by either a one-step or two-step gear pump. Within the gear pump, the gas and molten adhesive are thoroughly mixed and the gas is forced under pump outlet pressure into solution with the liquid adhesive. The pressurized liquid/gas adhesive solution is then supplied to a valved type dispensing gun from which the adhesive is dispensed at atmospheric pressure. Again, upon emerging from the outlet nozzle of the dispenser, the gas evolves from the solution in the form of small bubbles causing the adhesive to expand volumetrically and forming in an uncompressed state, a homogeneous solid foam having gas cells evenly distributed throughout the adhesive.
As set forth in the patents recited above, the methods for mixing the gaseous foaming agent with the molten adhesive and pressurizing the gas into solution in the adhesive is the use of a one or two-step gear pump. In this application, a molten adhesive and foaming gas flow into the interior of the gear pump where the meshing teeth of a pair of gears causes the gas and molten adhesive to be thoroughly mixed and the gas to be forced under pressure into solution to form a molten adhesive/gas solution. The gear pump is operable to increase the pressure of the gas and molten adhesive mixture to a pressure of approximately 300 pounds per square inch at which pressure the gas contained within the molten polymer is maintained in solution with the molten polymer, a condition in which it remains until dispensed at atmospheric pressure to form the foam. The intermeshing gear teeth of the pump operate as multiple small pistons to pull incoming liquid into the pump, pressurize it, and dispense it from the pump outlet. Hot melt adhesive compositions which have been foamed employing a gear pump as disclosed, for example, in U.S. Pat. No. 4,059,714 include conventional polyethylene-based hot melt adhesive compositions, such as Eastabond A-3 and A-32 manufactured by Eastman Chemical Company. These materials range in viscosity from about 2,200 cps to 20,000-35,000 cps at the usual dispensing temperatures of about 350.degree. to 400.degree. F.
It has been found, however, that when it is attempted to foam relatively high viscosity polymeric materials such as thermoset sealant materials having viscosities in the range of 50,000 to above 1,000,000 cps, a gear pump system becomes unacceptable for a number of reasons including inadequate mixing of the gas and polymer, unacceptable temperature rise of the polymeric materials, and reduced throughput. The problem of inadequate mixing is somehwat complex. First, since the viscosity of air or the gas to be mixed into the polymer is essentially zero, and the viscosity of the polymer quite high, the mixing of the one very low viscosity material into another very high viscosity material is difficult. Second, since the viscosity of the material is quite high, there are large line losses involved in moving the material through pumps, hoses, pipes, and the like making use of a recirculation system to increase mixing unacceptable. Third, because of the problem of temperature increase of the polymeric material, as discussed below, the addition of mixing or pumping devices to the system, which impart energy to the polymer, is generally not an acceptable solution to the problem of large line losses.
Some temperature rise is tolerable with some thermoplastic resins, for example, when foaming thermoplastics such as polyisobutylene-based materials and polyethylene-based hot melts but for thermosetting materials such as silicone RTV (room temperature vulcanizing) rubbers such temperature rise results in premature curing of the material giving it very short "open time" or even causing its setting up in the foaming equipment causing equipment stoppage. Likewise, such temperature rise can cause degradation of the polymer depending on its chemical structure or premature foaming in the system because of the increase in vapor pressure of the gas with temperature increase.
Investigation of the cause of the unacceptable throughput rates and temperature rise when attempting to foam high viscosity polymer materials using a gear pump has revealed that the action or mechanical work of the pump on the polymer material is converted to heat which raises the temperature of the polymeric material. As stated, a temperature rise as observed makes foaming of such relatively high viscosity materials using a conventional gear pump to be commercially impracticable.
Still further, in addition to the problem of overcoming large line losses in the system due to the nature of such high viscosity materials, there is also the problem of starving the input to the gear pump. In other words, the normal suction generated at the input of a gear pump is inadequate to draw sufficient quantities of such high viscosity materials into the pump to provide adequate throughput.